Work vanilla into the sugar, using a pastry blender or clean hands, until well distributed. Sift the cocoa into a separate, very large bowl; blend in the salt and milk powder. Gradually stir the sugar mixture into the cocoa until well mixed. The color should be uniform throughout. Store in an airtight container. To make one serving of cocoa place 1/2-cup of mix in a large mug, gradually add one-cup very hot (180-200ºF) water while stirring to dissolve mix. Top with marshmallows or whipped cream if desired.
The quantities of any of the ingredients can be adjusted to taste to make a drink that is more or less sweet, chocolate-y or milky. Non-instant dry milk powder can be substituted—use enough to make ten quarts liquid milk. To reconstitute mix when using non-instant milk powder use about 1/4-cup mix per cup of hot water or to taste. Non-instant milk powder is slower to dissolve in liquid and may form lumps if not stirred carefully. Add just a small amount of water at first and stir to make a smooth paste, and then gradually stir in remaining hot water.